1. Field of the Invention
The present invention relates to a semiconductor device, and particularly relates to a semiconductor device that is preferable for high frequency power application.
2. Description of the Related Art
For high frequency power application, such as power amplification at the final output stage of a cellular phone for example, an HBT (heterojunction bipolar transistor) having a heterojunction of InGaP/GaAs is used. Improvement in efficiency thereof as a power amplifier is important in a field such as cellular phones because it leads to extension of a possible talk time. In order to improve efficiency in a power amplifier using the HBT, it is effective to decrease so-called knee voltage (voltage between a collector and an emitter generated when collector current rising from zero changes to an almost constant value) among characteristics of the voltage between a collector and an emitter versus collector current of the HBT. Generally, the knee voltage can be reduced by decreasing emitter resistance and collector resistance.
Particularly, regarding devices for power application, there are many cases where a plurality of elements formed on a semiconductor substrate are connected in parallel to be used as one device, and in such cases, thermal runaway easily occurs when the emitter resistance is reduced. Thermal runaway is a phenomenon such that when a temperature increases by heat generated by a device or the like, current begins to concentrate in an element having a slightly higher temperature, and thus the element further generates heat and current concentrates more therein, thereby leading to failure or destruction.
In order to suppress the thermal runaway, a structure that inserts resistance (ballast resistance) in a base of each element is effective, but on the other hand, the inserted resistance causes deterioration of frequency characteristics. Techniques for achieving a balance between acquisition of frequency characteristics and suppression of thermal runaway include, for example, one described in Patent document 1 listed below. In this technique, an electrode for inputting an RF signal to a base and an electrode for applying bias voltage thereto are provided separately, thereby allowing a base layer extension portion, extending from the electrode for applying bias to a base layer, to function as the ballast resistance. This configuration increases a capacity between a base and a collector by extension of the base layer, and thus it is conceivable that acquisition of further frequency characteristics is difficult.    [Patent document 1] Japanese Patent Laid-open Application No. 2001-127071